As far as I know the german e-passport function does provide good way already. You basically use your passport to make a corresponding app only send the information “over ‘certain age’ or under”, technically no information needs to go to the government of when and where you try to vefify your age since it can all be done locally with your passport. The app is also open source if i recall correctly. It would definitely be a better option than any third party age verification but its not really used at all.

But i am not too familar with the actual working procedure of this function so it may not be entirely accurate.